1. Field of the Invention
The present invention relates to a color light receiving device and an image pickup device.
2. Description of the Related Art
As conventional light receiving devices, metal oxide semiconductor (MOS) capacitors and pn junction diodes made of semiconductors such as crystalline silicon, amorphous silicon, GaAs, or the like, are generally used. These light receiving devices can also be utilized as photoelectric conversion sections in image pickup devices, such as charge coupled devices (CCD) and complementary metal oxide semiconductors (CMOS). Although CCD and CMOS sensors are different from each other in signal reading-out method, however, for either image pickup device, a light receiving device having the same configuration can generally be used. Such an image pickup device is used for a variety of applications, including image cameras, copying machines, and facsimile machines.
However, conventional light receiving devices have had only the function to convert the intensity of light to an electric signal, having no ability to detect any particular color. For this a single-plate method is used. This method provides a color signal by covering individual photosensors with a monochromatic color filter, and combining the signal from a photosensor with those from adjacent photosensors, which are dedicated to other colors. As colors of the color filters, red (hereafter, may be expressed as R), green (hereafter, may be expressed as G), and blue (hereafter, may be expressed as B) are used as the three primary colors of light. Or, as complementary colors thereof, cyan (hereafter, may be expressed as C), magenta (hereafter, may be expressed as M), and yellow (hereafter, may be expressed as Y) are used. The method which adds green, having a wide visible region, to the three complementary colors to provide 4 colors has also been proposed.
For high image quality applications, a multiplate method, which separates a color image with a color separation prism and uses three or four image pickup devices, is employed. In an example, after the incident light is color-separated with a prism, the R, G, B three colors are sensed by respective photosensors. Further, a four-plate method, which adopts two photosensors for G in order to enhance the resolution, is also known.
However, the single-plate method has several problems.
A first of the problems is that a part of the light is absorbed by the color filter, resulting in the sensitivity being lowered. Especially, when the light is passed through a red color filter, the blue color and the green color are lost in the color filter, and thus only one third of the light is utilized at most.
A second of the problems is that, because the RGB three colors are detected in different locations, color separation is caused, and false color tends to be produced. Especially, false color tends to occur at the boundary between light and shade of the subject, and the false color can cause a moire phenomenon. To avoid the problem of such a false color or a moire phenomenon, an optical low-pass filter is used, however, the optical low-pass filter lowers the resolution. With an optical low-pass filter, the thinner the filter the lower the reduction in resolution will be, however, the effectiveness is also lowered, and thus reciprocal characteristics are exhibited.
The multiplate method also presents problems. The multiplate method requires a high precision prism and a color separation film (a dichroic mirror), and requires a highly accurate alignment technique to be used, which results in the cost and the size of the apparatus being increased. In addition, the loss of light in the prism and other optical elements is also a critical problem.
In order to eliminate such a false color problem, a configuration of a laminated type image sensor has been proposed (for example, refer to U.S. Pat. No. 5,965,875 and Japanese Patent Application Laid-Open (JP-A) No. 7-38136). If light receiving devices having sensitivity to different colors can be laminated, the light can be separated into respective colors in the same (planar) location, thus the problem of false color due to the difference in light receiving location can be avoided. With such a configuration, the low-pass filter is theoretically not needed.
The laminated type image sensor proposed comprises laminated light receiving sections, having a configuration in which the wavelength dependency of the absorption factor of Si is utilized for color separation in the direction of the depth thereof. This image pickup device is effective against a false color. However, this image pickup device carries out color separation on the basis of the depth of light entering into the Si, thus the spectral range detected by the respective light receiving sections is broad, which has resulted in insufficient color separation. This causes reduction in the amount of light converted to a signal in practice, with the sensitivity being lowered. In addition, there occurs a problem that, although a certain degree of color separation can be provided by design of the depth of the P-N junction in the respective light receiving sections, the design involves a trade-off relationship between the sensitivity and the color separation. That is if the color separation is enhanced, the sensitivity is decreased, for example. Therefore, this design has inherent limitations.
Color separation by the laminated type image sensor is insufficiently performed especially between blue and green colors. To solve this problem, a method which provides a green color sensor on or above the top of the Si substrate has been proposed (for example, refer to JP-A No. 2003-332551).
Therefore, there is a need in the art for a laminated type image sensor which generates no false colors, is an excellent light receiving device, and a provides enhanced sensitivity and color separation.